Well I am really really new when it comes to any type of sewing machine repair...but I am proud to say I fixed tension problems on my new to me 301A. Totally different machine I know : )
Here is a link that might help you with the bobbin case-
http://vssmb.blogspot.com/2011/07/20...g-ring-re.html
I took apart my tension unit, cleaned it and put it back together. At first I was having trouble getting the bobbin tension correct. I took it out and cleaned it too. Eventually out of frustration I tightened the screw all the way and it seemed to fix the problem. However, a few days ago I got to thinking about it and it seemed to me that having the screw tightened all the way couldn't be my real fix, I figured it was like fixing my tension by having the thread tension set on 0 or 9. So I took out the bobbin case screw, which is very tiny and easy to drop. I screwed it back in and voila, with a tiny bit of adjustment the tension is now perfect.